px4_work_queue: command line status output and shutdown empty queues

* adds a work_queue systemcmd that will bring a tree view of all active work queues and work items
 * WorkQueues now track attached WorkItems and will shutdown when the last WorkItem is detached

src/modules/fw_pos_control_l1/FixedwingPositionControl.hpp

@@ -187,7 +187,6 @@ private:
 	SubscriptionData<vehicle_acceleration_s>	_vehicle_acceleration_sub{ORB_ID(vehicle_acceleration)};
 
 	perf_counter_t	_loop_perf;				///< loop performance counter */
-	perf_counter_t	_loop_interval_perf;			///< loop interval performance counter */
 
 	float	_hold_alt{0.0f};				///< hold altitude for altitude mode */
 	float	_takeoff_ground_alt{0.0f};			///< ground altitude at which plane was launched */